1. Field of the Invention
This invention relates generally to clothes washing machines, and more particularly, to a method and system for controlling rotation of a washing machine tub.
2. Description of Related Art
Residential and commercial clothes washing machines are well known. A generally cylindrical tub or basket for holding the clothing and other articles to be washed is rotatably mounted within a cabinet. Typically, an electric motor drives the tub. During a wash cycle, water and detergent or soap are forced through the clothes to wash them. The detergent is rinsed from the clothes, then, during one or more spin cycles, the water is extracted from the clothes by spinning the tub.
One way of categorizing washing machines is by the orientation of the washing machine tub. Conventional, vertical-axis washing machines have the tub or basket situated to spin about a vertical axis. Articles to be washed are loaded into the tub through a door, which is usually situated on the top of the washing machine. A vertical-axis washing machine tub includes an agitator situated therein, which cleans clothes by pushing and pulling them down into the water. A motor typically drives the agitator, in addition to spinning the vertically-oriented tub during spin cycles. The motor usually operates at a constant speed, and a series of gears or belts are configured to drive the proper component at the proper time during each washing machine cycle.
Horizontal-axis washing machines, having the tub oriented to spin about an essentially horizontal axis, do not include an agitator, and a variable-speed motor drives the tub. During wash cycles, the tub of the horizontal-axis washing machines rotates at a relatively low speed. The rotation speed of the tub is such that clothes are lifted up out of the water, using baffles distributed about the tub, then dropped back into the water as the tub revolves.
Both vertical and horizontal-axis washing machines extract water from clothes by spinning the tub, such that centrifugal force extracts water from the clothes. It is desirable to spin the tub at a high speed and extract the maximum amount of water from the clothes in the shortest possible time, thus saving time and energy. The distribution of the clothes about the periphery of the tub affects the washing machine""s ability to spin the tub at a high speed.
Vertical-axis washing machines are especially susceptible to imbalance problems. Several factors contribute to this predicament. For instance, when a wash or rinse cycle completes and the water is drained from the tub, the clothes are gathered at the bottom of the tub, not distributed about the entire tub. In conventional washing machines, the tub typically is not perfectly cylindrical; but rather, includes a draft. When the tub spins, the clothes will xe2x80x9ccreepxe2x80x9d up the sides of the tub. However, since a constant speed motor typically drives the vertically-oriented tub, the motor quickly ramps the tub up to the full spin speed. There is little chance for the clothes to distribute about the periphery of the tub, so they creep up the tub""s sides in an unbalanced fashion.
The unbalanced, spinning tub vibrates within the cabinet. In conventional vertical-axis washing machines, if the vibration is too severe, the tub will trip a switch mounted inside the cabinet, stopping the tub""s rotation and activating a tub-imbalance alarm. A user then manually redistributes the wet clothes within the tub, and restarts the spin cycle.
Horizontal-axis washing machines typically are less vulnerable to tub imbalances. As discussed above, the tub in a horizontal-axis machine is driven by a variable speed motor. This allows the inclusion of a xe2x80x9cdistributionxe2x80x9d cycle, wherein the tub is rotated faster than the rotation speed of a wash cycle, but slower than in a spin cycle. The tub rotation speed is gradually increased, until the clothes begin to xe2x80x9cstickxe2x80x9d to the sides of the tub due to centrifugal force. The slower rotation speed allows the clothes to more evenly distribute about the sides of the tub. Once the clothes have been distributed about the tub, the speed is increased to a full spin speed to extract the water from the clothes.
Even though horizontal-axis washing machines may be less prone to tub imbalances, they are not immune to tub imbalance problems. If the clothes do not evenly distribute during the distribution cycle, the unbalanced load within the tub will cause unwanted vibrations as the tub rotates. Rather than applying all of the motor""s power to spinning the tub at the highest possible speed, power is wasted in tub movement and cabinet vibrations.
Controlling the speed of the rotating tub is also important to optimize washing machine performance. In horizontal-axis washing machines, the tub must be rotated at the proper speed in order to carry out the functions of the various machine cycles and keep machine noise at a minimum. Further, it is desirable to change the rotation speed of the various cycles in a controlled manner. However, the load contained within a washing machine tub can vary widely, due to different wash load sizes, the type items being washed, the amount of water contained within the tub, etc. Thus, the torque required to achieve a desired tub rotation speed also varies widely, complicating speed control of the rotating tub.
The present invention addresses these, and other, shortcomings associated with the prior art.
In one aspect of the invention, systems and methods of controlling the rotation speed of a washing machine tub are disclosed. The novel control systems and methods of the present invention employ an adaptive gain control technique, wherein controller gain factors are adjusted as a function of one or more operating parameters of the washing machine system.
One method includes receiving an indication of the actual tub rotation speed and comparing the actual tub rotation speed to a set point speed to calculate a speed error. In response to the speed error, at least one gain factor is determined and applied to the speed error to calculate a correction factor. The gain factor may include one or more of a proportional, integral and/or derivative gain factor. In certain embodiments of the invention, a torque demand factor is determined based on the speed error and the gain factor is determined in response to the torque demand factor. Further, the average torque demand may be calculated, and the gain factor is determined based on the average torque demand factor.
The tub rotation speed may be changed in a controlled manner. In one implementation, a demand gap value is determined by comparing the difference between a demanded rotation speed and the set point speed. The demand gap value is compared to a predetermined maximum speed change value, and the set point speed is increased by the lesser of the demand gap value and the maximum speed change value.
In accordance with other aspects of the invention, the size of the load in the washing machine tub is determined, and the gain factor is adjusted based on the load size. The desired tub rotation speed may also be varied based on the load size. Several parameters may be considered for a determination of load size, including the average system torque demand.
Moreover, the adaptive gain control techniques may be selectively employed. In an exemplary embodiment of the invention, if the washing machine is operating in a spin cycle, the gain factor comprises a predetermined gain factor, and if the washing machine is operating in a cycle other than a spin cycle, the gain factor is determined in response to system parameters such as the speed error.
In still other aspects of the invention, a system for implementing the disclosed control methods includes a speed detection device operable to output an indication of the tub rotation speed, a controller coupled to the speed detection device to receive the indication of the tub rotation speed, and a memory accessible by the controller and storing a speed demand value. The controller is operable to compare the tub rotation speed to the speed demand to calculate a speed error and apply a gain factor to the speed error to calculate a correction factor. The gain factor is determined based on system parameters, including the size of a load contained within the washing machine tub, the tub rotation speed, the speed error, the speed demand, etc.
In accordance with further aspects of the invention, a clothes washing system includes a cabinet, a tub rotatably mounted within the cabinet, a motor operably coupled to the tub for rotating the tub within the cabinet and a speed detection device operable to output an indication of the tub rotation speed. A controller is coupled to the speed detection device to receive the indication of the tub rotation speed, and a memory storing a speed demand value is accessible by the controller. The controller is operable to compare the tub rotation speed to the speed demand to calculate a speed error and apply at least one gain factor to the speed error to calculate a correction factor. The gain factor is determined based on an indication of a load contained within the washing machine tub and/or on other system parameters.